I did some quick looking around, but my PHP skills are pretty nascent. I am sure there is just a package that needs installing.
Edit to add additional information about the setup:
yum info php
I get the version of PHP as 5.1.6:
Name : php Arch : i386 Version : 5.1.6
More information, this is a VM running CentOS at GoDaddy:
CentOS release 5.4 (Final)